Test-Driven Development - TDD - is a great way to catch bugs before they go live, to ensure fixes stay fixed, and to prove the functionality of software. This stability alone is reason enough to ensure tests exist for as much of every application as is feasible, but is not the only benefit.
When you write the all the key tests for a piece of functionality in advance of writing the rest of the code, you are defining when that functionality will be complete, how much progress has been made, and specifically what tasks are next to be done.
For anyone practising Agile development, the parallels should be blatant. There is similarity in the benefits too, particularly with respect to keeping focused on the task being worked on and knowing when you're done and ready to move on.
With Agile development, each piece of functionality is broken down in a couple of ways - as business-focused acceptance criteria, and development-focused tasks, which is (in part) about making large pieces of work less daunting. Likewise, TDD provides a way for an otherwise overwhelming piece of work to be described step-by-step by someone who understands the matter at hand, and (with appropriate software structure and functionality) can then be divided up amongst multiple developers to increase overall velocity.
Incomplete functionality can be more easily passed from one developer to another, simplifying the explanations of what has been done and still needs to be done; the tests do the talking.
TDD means all newly written code has tests to describe what it should do, which helps to build trust in the application - a full set of passing tests proves a particular change has not altered the tested functionality, and allows you to deploy it with far more confidence than otherwise.
TDD means writing the tests happens first - before making the changes that the tests apply to, not afterwards. Writing tests afterwards still has advantages compared to no tests at all, but it does not guarantee that all code has tests - indeed, it makes it more likely that tests will be rushed or dropped if time is limited; precisely the situation when bugs are more likely to be introduced - and thus can give false confidence in what the result of a set of changes may be.
Test-Driven Development increases productivity. As with most things, TDD has a learning curve - it's not something a programmer will instantly be completely efficient with - but some programmers will still complain (erroneously) that writing automated tests slows them down.
It's obvious that writing tests is additional work compared to not doing so, but this additional upfront cost comes with the saving of not having to manually step through performing repetitive tasks with minor variations each time - each test is written once, and is executed easily each time the code progresses. When automated, tests are not forgotton - the full test suite can be run at any time to check for regressions or unexpected side-effects. Automated testing doesn't remove the need to use an application as a user, but it far reduces the time it takes to check that changes are working as intended, and helps reduce the space for human error.
Of course, practising TDD does mean that pesky bugs are identified at the time of development - not days, weeks, or months down the line - and this can also appear to make things slower; in reality identifying bugs sooner generally makes the issue easier to fix.
Finally, with TDD you know when you're done - when all the tests pass. Without tests written up-front, there may not be a clear finishing point. It's easy for developers to become side-tracked, or to unconciously introduce scope creep, and thus slow down the completion of a task which an appropriate set of test cases would have identified as already implemented.
This article hopefully explains how Test-Driven Development is not just about code quality, reducing application errors and increasing stability. It enables developers to work smarter and faster, give clearer progress updates, know what they're doing next and not become disheartened on bigger chunks of work.
TDD is a critical methodology which any competent professional developer should
want to use.